1. Field of the Invention
This invention relates to a data communication system to perform data communication between a host and a device.
2. Description of the Related Art
FIG. 26 shows a structure of a conventional data communication system for a car audio. In the system, data communication between a microcomputer 100 and each of four ICs (Integrated Circuits) is performed through four communication lines, that are a control line (CE) 111, a clock line (CL) 112, a data input line (DI) 113 and a data output line (DO) 114. The control line 111 provides a control signal CE and the clock line 112 provides a clock CL. The data input line 113 is used to input the data into the ICs, while the data output line 114 is used to output the data from the ICs. The four ICs are a display driver IC 115, a key scan IC 116, a DSP (Digital Signal Processor) 117 and an RDS/DARC (Radio Data System/Data Radio Channel) 118. Between the key scan IC 116 and the microcomputer 100, in particular, serial data communication is performed through the four lines described above.
This kind of data communication system has been adopted in a very large number of electronic equipment, since it enables detecting a data read-in request signal (a signal to request reading-in of key data stored in the key scan IC 116, for example) using the data output line 114, that is, transmitting the data read-in request signal to the microcomputer 100, as well as realizing very simple data communication.
However, with the number of data lines being fixed to four, it is not a preferable communication method when there is a limitation on the number of input/output ports of the microcomputer 100 or there is a limitation on the number of signal lines drawn on a circuit board such as a printed circuit board or a flexible board or on the number of signal lines going through a connector.
In order to reduce the number of communication lines, there has been contrived a three-line data communication system having the control line, the clock line and a data input/output line that consolidates the data input line 113 and the data output line 114 (Refer to Japanese Patent Application Publication No. H09-238150.). However, there has been a problem that it is not possible to receive an address for inputting the serial data and an address for outputting the serial data while the data read-in request signal is outputted to the data input/output line, because an electric potential of the data input/output line is fixed at an L (low) level when the control signal CE is at the L level.
In order to solve the problem, a function of the data read-in request signal may be removed, or a terminal dedicated to output the data read-in request signal may be provided. However, enabling both the data transmission/reception function and the detection of the data read-in request signal eventually requires four communication lines which are not preferable in the case where there is the limitation on the number of communication lines as described above.
In order to solve the problem described above, there has been contrived another three-line data communication system in which the data read-in request signal is outputted to the data input/output line (Refer to Japanese Patent Application Publication No. H09-238151). However, because a control method to transmit the address to the data input/output line (a method to turn the data input/output line into a high impedance state) is limited to changing the clock CL (from the L level to an H (high) level or from the H level to the L level), there is a possibility that the data input/output line might be turned to the high impedance by error to hinder accurate detection of the data read-in request signal when a noise or the like is induced in the data input/output line.
As described above, there has been required a data communication system that enables performing both data communication and detection of the data read-in request signal while reducing the number of communication lines from the conventional four, and accurately detecting the data read-in request signal even when the noise is induced in the data line.